1. Technical Field
This invention relates generally to monobloc pistons for diesel engine applications, and more particularly to the construction of oil cooling galleries within monobloc pistons.
2. Related Art
In diesel engine applications, either so-called monobloc piston assemblies or articulated piston assemblies are typically used. Articulated pistons have a piston head with a separate skirt that is free to pivot relative to the piston head throughout a stroke of the piston, whereas the monobloc pistons have a piston head unitized with a skirt, such that the skirt is immovable relative to the head. Due to the severe heat that is generated in diesel engine applications, regardless of whether an articulated or monobloc construction is used, typically an oil cooling gallery is provided in the piston head to facilitate cooling the piston head.
In an articulated piston construction, the oil cooling gallery is typically provided having an “open-bottomed” construction. An opened-bottomed construction is advantageous in that oil is free to enter the oil cooling gallery from a sweeping range of angles throughout a stroke of the piston. This allows the oil to hit various portions of the oil gallery, thereby enhancing the cooling effect to the piston head. As such, with an open bottomed gallery, the orientation of oil jets responsible for injecting oil toward the gallery is not of critical concern. Typically, the oil jets are arranged at an angle relative to a longitudinal axis of the piston assembly to provide the sweeping range of angle or pattern of oil directed toward and into the oil gallery.
In contrast to the articulated construction having an open-bottomed gallery, the monobloc construction typically incorporates a bottom wall from which the unitized skirt depends, and wherein the bottom wall substantially encloses the oil gallery. The bottom wall provides the structural integrity desired to inhibit unwanted deflection of the unitized skirt that could lessen the useful life of the piston assembly. To provide for oil flow into and out of the substantially closed oil gallery, generally circular openings are typically provided in the bottom wall. As a result of having generally circular openings, the orientation of the oil jets must be directed so that the oil can enter the oil gallery, preferably throughout the stroke of the piston. This requires the oil jets to be oriented generally vertically, or parallel to the longitudinal axis of the piston, so that the oil is injected directly into the generally circular opening in the bottom wall.
As a result of the differences between the two different oil gallery constructions above, and particularly with regard to the injection of oil into the oil galleries, the two different constructions are largely incompatible with one another. As such, in service, it is necessary to replace articulated pistons with articulated pistons, and monobloc pistons with monobloc pistons. If a monobloc piston with the above construction were interchanged with an articulated piston with the above construction, inadequate oil flow into the oil gallery of the monobloc piston would result. This is due to the arrangement of the oil jets with the two different constructions. A pre-existing angled oil jet used in an articulated piston application would not be able to direct oil into the generally circular opening of a monobloc piston, thus causing the monobloc piston to overheat and seize within the cylinder bore. On the other hand, if an articulated piston were incorporated in a monobloc piston application, the advantageous sweeping pattern of oil would be lost due to having a vertically arranged oil jet. This too would likely lead to a reduced life for the articulated piston assembly.